Parallel operation of power generator units is used to share loads of a utility grid or an island system amongst two or more units. An increased power rating can be met by paralleling two lower power units so that there is no need for one single high power unit. Parallel operation of several units allows routine maintenance of one unit without having to completely shut down the system, since other power generators can temporarily take over the load from the one unit being stopped for maintenance. As in any parallel operation, redundancy of power generation increases the availability of power in the utility grid.
While in grid mode, with the various power generator units connected to a utility grid with stable voltage and frequencies, the operation of several power generation units in parallel is less critical, but operation of parallel connected power generator units in islanding is still challenging. Since the power generation units can be located quite far apart with significant line impedance between them, parallel operation of several units should be achieved with no or minimum control communication.
In the past, various attempts to control parallel power generator units have been made.
One unit can be connected to batteries and can be defined as the master that controls the island bus voltage and frequency to a set value. All other units operate in grid mode. Any change in load is detected and taken by the master unit with the help of battery backup for load jump support. The master unit can dip the system frequency and the output voltage to communicate with other units about the active and reactive loads respectively. The control circuits of the other units can measure the frequency and the voltage values of the island network, and calculate the active and reactive power references respectively and generate the same. As the other units pick up the power, the master unit decreases its generation and maintains the system voltage and frequency. The process will go on until there is stability in the entire system. Even though the control with a battery backed master works well in most systems, the dynamic performance of this two stepped approach can be very slow.
In a variation, rather than being synchronized to the master unit, all of the parallel operated units, including the battery-backed master unit, are synchronized to the island network with a phase locked loop (PLL), a closed loop frequency control system, in which functioning can be based on the phase sensitive detection of a phase difference between the input and output signals. The PLL synchronization signal which can be distributed to all units can be taken from an external sine wave generator. To avoid wires, the sine wave can be communicated over a wireless connection.
In another variation, instead of using an external sine wave generator, a master unit sets the system frequency to a constant value. All other units in parallel can lock to that frequency with a PLL. The so-called droop method can be used to stabilize the system. Active power vs. frequency-droops and reactive power vs. voltage-droops are based on the fact that for inverters, the active power P is predominantly dependent on the phase angle φ between the output voltage of the inverter and the load voltage, while the reactive power Q mostly depends on the output-voltage amplitude E. With droop coefficients m and n the following equations or droops can be described:ω=ω*−m P (ω*=frequency at no load)E=E*−n Q (E*=output voltage at no load)
One possible approach for parallel operation of voltage source converters in islanding without communication has been proposed by “Parallel Operation of Voltage Source Inverters”, T. Kawabata et al., IEEE Transactions on Industry Applications, Vol 24, No. 2, March/April, 1988. Active power vs. frequency and reactive power vs. voltage droops are introduced to share the active and reactive power.
Parallel operation with current minor loops but with communication of total load current has been proposed by “Parallel U.P.S. with an Instantaneous Current Sharing Control”, Jang-Sik Yoo, et al., IECON, IEEE, pp. 568˜573, 1998. In this case the total load current is measured, is divided with number of units in operation and then fed-forward to each inner current controller.
Another method for controlling parallel operation of converters has been disclosed in “Control of Parallel Inverters in Distributed AC Power Systems with Consideration of Line Impedance Effect”, Anil Tuladhar et al., IEEE Transactions on Industrial Applications, Vol 36, No. 1, January/February 2000, with communication through the power network by injecting special frequency signals for communication.
In “Distributed Uninterruptible Power Supply Systems” by Mukul C. Chandorkar of the University of Wisconsin-Madison, USA (Dissertation for the degree of Doctor of Philosophy—Electrical Engineering—1995), which is incorporated hereto by reference in its entirety, the operation and control of distributed networks and uninterruptible power supply (UPS) systems are described in detail. In Chapter 2.3.1, the parallel operation of voltage source inverters is described, making use of current minor loop on page 30. The system is shown in FIG. 2.16 on page 30. On page 31, second paragraph, the author notes that when applying a current control scheme, it is mandatory to have a communication link between invertors running in parallel. Otherwise the current controllers of different parallel units will interact to make the control scheme unstable.
In “Control of Parallel Connected Inverters in Standalone AC Supply Systems” by the same author (Mukul C. Chandorkar) in an IEEE Transaction on Industry Applications, vol. 29, no. 1, January 1993, a control scheme for controlling inverters in a standalone system is described in which the entire AC power is delivered through inverters without any synchronous alternators. In the control system described therein, measured currents are fed back and used for power calculation and control. Current however is again not controlled in the described system, hence there are no inner current loops.